The Crystal (Testing) Method

It used to be any good electronics experimenter had a bag full of crystals because you never knew what frequency you might need. These days, you are likely to have far fewer because you usually just need one reference frequency and derive all the other frequencies from it. But how can you test a crystal? As [Mousa] points out in a recent video, you can’t test it with a multimeter.

His approach is simple: Monitor a function generator with an oscilloscope, but put the crystal under test in series. Then you move the frequency along until you see the voltage on the oscilloscope peak. That frequency should match the crystal’s operating frequency.

It is interesting that because of the resonance of the crystal, the voltage on the scope can be much higher than the input voltage from the signal generator. This is a simple test, but effective. Of course, you could also have a little oscillator and see what the crystal does in a real circuit.

We’ve tested crystals before with a network analyzer and we even made a video that shows essentially the same test [Mousa] uses, although it was on an LC circuit, not a crystal. You do need to be careful you aren’t operating the crystal in an overtone mode by accident, although presumably if it works on a harmonic, it should work on the fundamental frequency, too. We’ve also seen crystal testing and classification done with a software defined radio.

Why not put the crystal on channel A and the generator on channel B and set the scope to show A-B, then find the frequency at which the trace is closest to zero? (Or invert one of the channels and use A+B).

You could probably learn a lot about crystal oscillators from Leif’s project of building a low noise 14 MHz crystal oscillator with phase noise of -180 dBc/Hz @10kHz – http://www.sm5bsz.com/osc/newref.htm

In second of his technical videos for nerds only as he puts it, he measures 62 crystals, their serial resistance, main resonance frequency and spurs (spurious resonances) using a VNA. https://www.youtube.com/watch?v=e10BC32lutA

I used the same trick to measure the resonant frequency of a coil for a ferrite rod antenna.

For anybody who might have the same signal-generator, I once wrote a python-library to control that device remotely over USB: https://github.com/on1arf/jds6600_python
If you add some code to query the Vpp value of your oscilloscope, you can use this to write some python code to automate this process.

Or skip all that (including buying expensive equipment) and instead by a 1Hz-50MHz xtal tester/counter kit w/5 digits. Even Amazon carries it (from its store front) https://www.amazon.com/Frequency-1Hz-50MHz-Crystal-Counter-Housing/dp/B01L4BX6UO, which is even cheaper than Banggood’s price. I got one a year ago and it did ok on all of my crystals except 4MHz (some of their documents indicated there is a dead spot around 4MHz, but not why). There are also a bunch of YouTube videos on this.

This video did not “mention” how to test series vs. parallel resonant crystals, which can result in a significant frequency difference and possibly different aging and life, especially if over driven in an oscillator.